Engine systems may be configured with boosting devices, such as turbochargers or superchargers, for providing a boosted aircharge and improving peak power outputs. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits. However, compressors are prone to surge. For example, when an operator tips-out of an accelerator pedal, an engine intake throttle closes, leading to reduced forward flow through the compressor, potentially causing compressor surge. Surge can lead to NVH issues such as undesirable noise from the engine intake system. As such, based on the state of the airflow and induction pressure, the compressor may be in a soft surge or hard surge operating region. During hard surge, the compressor allows air to momentarily backflow through the compressor resulting in rapid pressure oscillations. During soft surge, there is relatively smaller instability of compressor operation.
To address either form of compressor surge, engine systems may include a compressor bypass valve coupled across the compressor to enable rapid decaying of boost pressure. One example of such a compressor bypass valve (also known as a compressor recirculation valve) is shown by Blaiklock et al. in US 2012/0014812. Therein, the compressor bypass valve is an open/closed type of valve that is maintained closed during steady-state engine operation and actuated open in response to any indication of surge. By opening the valve, a portion of air discharged from the compressor is recirculated to the compressor inlet. Still other approaches may involve the use of a fully variable valve to address surge.
However the inventors herein have identified potential issues with such systems. As one example, the valve of Blaiklock is kept closed when there is no indication of surge to reduce energy wastage and fuel consumption. In particular, if the valve is kept open (for example to alleviate surge tendencies before surge actually occurs), the compressor may not be able to provide the boost pressure required to meet the demanded engine torque. Fuel economy is also degraded due to extra compressor work needing to be compensated by increased turbine work. Also, energy may be required to hold the valve in the open position. The inventors have realized that such an approach may actually lead to more energy being wasted. This is because the margin to the surge limit may be smaller during steady-state conditions when the valve is kept closed, causing frequent surge, and requiring the valve to be frequently opened. As such, this consumes energy. In addition, if surge is caused due to an operator tip-out event, which is soon followed by an operator tip-in event, energy is needed again to actuate the valve closed and increase boost pressure. This can degrade torque responsiveness. In particular, if surge is avoided by opening the valve in response to a tip-out event which is soon followed by a tip-in event, it will take more time to recover boost pressure and provide engine torque than if the valve was not opened completely.
Further still, the valve of Blaiklock may not be able to address hard surge and soft surge equally well. For example, the opening of the compressor bypass valve and the resulting rapid increase in compressor flow may be better at addressing hard surge, which occurs during transient conditions such as tip-out, but not soft surge which can occur during steady state conditions. In particular, the large induction pressure reduction associated with the valve opening may decrease engine performance during the soft surge conditions if the valve is commanded open.
In view of these issues, the inventors herein have recognized that it may be advantageous to maintain the valve partially open and recirculate at least some compressor flow during steady-state conditions, before an indication of surge is received, even if a small amount of energy is consumed in doing so. In one example, the above issues are addressed by a method for a boosted engine having a compressor recirculation valve that can be shifted between a plurality of positions including at least a fully open position, a fully closed position, and a semi-open position. In one embodiment, the method comprises, operating an engine, without surge, with a valve coupling a compressor outlet to a compressor inlet at a semi-open position; in response to an indication of less severe surge or no surge, maintaining the valve at the semi-open position and in response to an indication of more severe surge, shifting the valve from the semi-open position to a fully-open position. In this way, both hard and soft surge can be addressed with reduced energy usage and while improving torque responsiveness.
For example, a boosted engine system may include a compressor recirculation valve positioned in a passage coupling the compressor outlet to the compressor inlet. The valve may have a default semi-open position during steady-state boosted operating conditions. The valve may be passively maintained in the semi-open position, without actuating an external actuator coupled to the valve, by a pair of opposing springs that are pre-loaded and pre-compressed. The pre-loaded springs hold the valve in the default semi-open position and enable a nominal flow rate to be provided across the compressor. As such, this increases the margin to a hard and soft surge limit, reducing the propensity for surge, and thereby reducing the need for valve actuation. By reducing the need for valve actuation while improving the surge margin during the steady-state conditions, energy usage is reduced. The valve may be maintained in the default semi-open position to reduce the occurrence of soft surge. In response to an indication of hard surge (such as during a tip-out), the external actuator may be energized in the opening direction to move the valve from the semi-open position to a fully-open position. By opening the valve to a larger degree, a rapid increase in compressor flow rate is provided to address the hard surge substantially immediately. In comparison, in response to a sudden increase in torque demand (such as during a tip-in), the external actuator may be energized in the closing direction to move the valve from the semi-open position to a fully-closed position. Herein, while engine power is being rapidly increased, the valve is closed to enable a rapid increase in boost pressure.
In this way, by using a compressor recirculation valve that is partially open during steady-state conditions, a margin to surge is improved and surge occurrence is reduced. Further, when surge (e.g., hard surge) does occur, the valve can be rapidly transitioned to a fully open position, thereby improving a hard surge response time. Likewise, the valve can be rapidly transitioned to a fully closed position to increase boost pressure, improving transient torque response and driveability. The incorporation of a semi-open mode substantially improves the margin to a hard and soft surge limit over conventional open/closed compressor surge valves, which have position limitations, without expending additional energy. This allows the three-state valve to better address both hard surge and soft surge. In addition, compared to fully variable valves, the three-state valve offers lower cost and complexity benefits. Further, the valve consumes substantially lower amounts of energy under most engine operating conditions. For example, the pressure balanced valve configured as an electronic throttle body may be fully variable but may use very little energy in any steady-state condition. Overall, surge can be addressed with a simpler valve.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.